I take it your building the Grendel on an AR15 platform. There are several ways you can go about it.
The easiest would be use a 1pc mount like a Burris P.E.P.R. scope mount or use an extended riser to mount standard scope rings on. It's hard to mount a scope forward enough without using these types of mounts on the upper rail.
I have a Burris mount on my 458 SOCOM,but I prefer to use a riser/rings.This type of mount raises the scope up too far off the rifle for my taste.
I used a Weaver Extended riser and medium Burris Signature Zee rings on my LR-260 AR10.